1. Field of the Invention
This invention relates to an exhaust gas purifying device for an internal combustion engine.
2. Description of the Related Art
Almost all internal combustion engines, depending on the engine type and fuels employed, discharge a variety of harmful gases in addition to carbon dioxide gas and steam. A relatively large quantity of harmful gas is discharged at the start of the engine--that is, when the engine cylinder temperature is low. Thus, it is necessary to provide an exhaust gas purifying means for an internal combustion engine, particularly for application when the engine cylinder temperature is low.
For instance, in the case of a diesel engine using methanol as its fuel, the discharge gas contains, among other organic substances, formaldehyde produced by incomplete combustion of the methanol fuel. Formaldehyde is not only malodorous but also injurious to the human body, and therefore it should not be released into the air. To prevent this release to the air, a method may be employed in which the formaldehyde and other organic substances are decomposed through oxidation by using a catalyst of noble metal such as platinum, palladium, cobalt or nickel before the substances are released into the air. However, this method is not substantially effective in the case where the exhaust gas temperature is lower than 100.degree., such as occurs during the warming up of the engine. This is because it requires a high temperature of about 200.degree. C. or higher to make the noble metal catalyst active.
Furthermore, organic substances such as formaldehyde and methanol not burned are discharged in relatively high amounts until the engine is warmed up. Accordingly, even if a catalyst is used, the exhaust gas is undesirably high in contaminants during warm up of the engine.
In order to overcome the above-described difficulties, Japanese Utility Patent Applications (OPI) Nos. 5820/1987 and 10223/1987 (the term "OPI" as used herein meaning an "unexamined published application") have disclosed a method in which an adsorbing device is arranged in the exhaust gas path between the catalyst device and the internal combustion engine, so as to adsorb the formaldehyde, methanol, and other substances which are discharged until the catalyst becomes active. The formaldehyde and other substances thus adsorbed are released from the adsorbing device as the exhaust gas temperature increases. However, by that time, i.e., before the formaldehyde and other substances are released, the temperatures of the catalyst become sufficiently elevated that the catalyst acts to decompose the organic substances. Thus, during warming up and thereafter too, release of the highly contaminated exhaust gas into the air can be prevented.
However, in the exhaust gas purifying device with the adsorbing device set before the catalyst device, the exhaust gas temperature is decreased because of the thermal capacity and surface heat radiation of the adsorbing device, and therefore the exhaust gas temperature at the inlet of the catalyst device is increased later than in the case where no adsorbing device is used upstream of the exhaust gas purifying device. That is, the adsorbing device for holding the harmful gas until the exhaust gas becomes high enough to make the catalyst active operates to delay the rise in temperature of the catalyst, thereby to delay the start of activity of the catalyst device.
Accordingly, an object of this invention is to eliminate the above-described difficulty accompanying a conventional exhaust gas purifying device. More specifically, an object of the invention is to provide an exhaust gas purifying device in which the catalyst becomes active more quickly than in prior art devices.